Some process control systems utilize a two-wire process control network. The two wires transmit analog or digital data signals between a control processor and field devices connected to the network, and are also connected to a power supply that supplies power to those field devices that are powered from the fieldbus network. Terminating impedances may be provided at the ends of the trunk line to avoid signal reflections.
Examples of known two-wire process control networks include, but are not limited to, Foundation Fieldbus H1 and Profibus PA fieldbus networks.
A two-wire process control network often includes a network trunk connected to the controller and spurs that extend from the network trunk and connect the field devices to the network trunk. The trunk and spurs may be arranged in different network topologies known in the art, including (but not limited to) point-to-point topology, trunk-and-spur topology, tree topology, and combinations thereof.
The spurs may be provided by a device coupler that includes a trunk interface or input connecting the device coupler to the network trunk and multiple spur interfaces or outputs that connect multiple spurs to the device coupler. One or more device couplers may be attached to the network trunk. If a device coupler is connected to an end of the network trunk, the device coupler may also provide the terminating impedance for that end of the trunk.
A field device located in a hazardous location (that is, a location where there is a risk of explosion caused by an electrical spark) may be connected to the network trunk by a spur having limited power delivery to the field device to reduce the explosion risk. The spur line may also be galvanically isolated from the network trunk. Galvanic isolation between two network segments breaks direct connections between the segments and so prevents the flow of electrical current between the network trunk and the spurs, further reducing the explosion risk.
Galvanic isolation between trunk and spur segments can also provide other benefits. Isolation reduces the likelihood of ground loops that may propagate transient voltage spikes between trunk and spur segments. Isolation may also prevent propagation of any common-mode noise between trunk and spur segments. That is, galvanic isolation can provide benefits even for network use in non-hazardous locations.
There are several conventional approaches to providing a device coupler having galvanic isolation between the trunk interface and multiple spur interfaces.
FIG. 2 illustrates a device coupler 210 that includes a trunk interface 212 and eight spur interfaces 214. An isolation element formed as a transformer 216 is connected to the trunk interface 212 and the spur interfaces 214. The transformer's primary winding 218 is connected to the trunk interface 212. The spur interfaces 214 are connected in parallel to the transformer's secondary winding 220. The transformer 216 provides isolation between the trunk interface 212 and all the spur interfaces 214; that is, all of the spur interfaces are in the same isolation set because all the spur interfaces are connected to the same transformer secondary winding 220.
FIG. 2 illustrates the device coupler 210 connected to a network trunk 222 and having spurs 224 connected to the spur interfaces labeled “S I 1” and “S I 2”. The spurs 224 are connected to field devices 226. Although the single transformer 216 isolates the field devices 226 from the network trunk 222, the field devices 226 are not galvanically isolated from one another. Current can flow between any of the spurs 224 due to all the spur interfaces 214 being connected in parallel with the transformer secondary winding 220.
A modification of the single-transformer approach is shown in FIG. 3, which shows a device coupler 230 connecting the two field devices 226 to the network trunk 222. The device coupler 230 is otherwise similar to the device coupler 210 except for the transformer 216 having multiple secondary windings 220 rather than a single secondary winding. Each secondary winding 220 is connected in series to a respective spur interface 214.
Like in the device coupler 210, the transformer 216 provides isolation between the trunk interface 212 and all the spur interfaces 214. But because each secondary winding 220 is connected to only one spur interface 214, the secondary windings 220 also provide galvanic isolation between the spur interfaces 214. Current cannot flow between the spurs 224 because the secondary windings 220 isolate the spur interfaces 214 from one another.
The isolation between adjacent spur interfaces 214, however, is less than the isolation between each spur interface 214 and the trunk interface 212 due to the spacing requirements of the multiple secondary windings 220.
Yet another approach to providing a device coupler having galvanic isolation between the trunk interface and multiple spur interface is shown in FIG. 4, which illustrates a device coupler 240 connecting the two field devices 226 to the network trunk 222. The device coupler 240 utilizes a modular system in which a separate transformer 216 is provided for each spur interface 214. The primary windings 218 of the transformers 216 are connected in parallel with the trunk interface 212 but the secondary winding 220 of each transformer 216 is connected in series with a respective spur interface 214. Each spur interface 214 is individually isolated from the trunk interface 212 by a dedicated transformer 216.
Each spur interface 214 is also isolated from another spur interface 214 by two transformers 216, that is, any current must pass through two transformers 216 to flow from one spur interface 214 to another spur interface 214. Current cannot flow between the two spurs 224 shown in FIG. 4 unless the current also flows through the two transformers 216 attached to the respective spurs 224.
Helfrick et al. U.S. Pat. No. 7,940,508 owned by the applicant herein and having common inventorship with this application discloses a variation of the modular system shown in FIG. 4. The '508 patent discloses connecting the trunk interface to a backplane. Separate spur interface modules attached to the backplane include a spur interface and an isolation element connected to the spur interface that also connects to the trunk interface through the backplane. Spur interface modules can be added or removed from the backplane as needed.